1. Field of the Invention
The present invention relates to an impulse sensor for sensing an inpulsive force when an impulsive force not less than a predetermined value is transmitted and, in particular, to an impulse sensor suitable for operating an air bag of an automobile.
2. Description of the Prior Art
A first example of a known prior art impulse sensor is shown in FIG. 6.
This first prior art impulse sensor, as shown in FIG. 6, comprises a weight 23 held in a case body 21 attracted by a magnet 22 disposed at one side of the case body 21, a cylindrical guide 24 for guiding the weight 23 and having an inner diameter slightly larger than a diameter of the weight 23, and a pair of contacts 25a and 25b provided on an inner surface of the other side of the case body 21 opposing the cylindrical guide 24.
When the impulse sensor is mounted on an automobile, normally, the weight 23 is attracted to and held by the magnet 22 so that the pair of contacts 25a and 25b are in a non-conductive condition. When the automobile stops suddenly from the normal moving condition, the weight 23 is caused to move to the left (in FIG. 6) due to an inertia force of the weight 23. At this time, the air in the case body 21 must pass through gaps between the inner surface of the cylindrical guide 24 and the outer surface of the weight 23. However, the weight 23 is prevented from reaching the pair of contacts 25a and 25b due to the viscosity of the air. On the other hand, when the automobile stops suddenly due to a collision, the weight 23 is pulled to the left by a velocity difference and by deceleration. Thus, when the impulsive force is sustained for a certain time period, the weight 23 overcomes the viscosity force of the air and reaches the pair of contacts 25a and 25b to render them conductive. As a result, a switch signal can be obtained.
Furthermore, a second prior art example of an acceleration sensor is disclosed in Japanese Patent Lain-Open publication No. 60-233564.
In this acceleration sensor, a magnetic fluid and a medium which is not mixed with the magnetic fluid and which has a specific gravity different from that of the magnetic fluid are sealed in a case, and a constant magnetism generating means is provided to apply a constant magnetic field to the magnetic fluid to form the magnetic fluid in a cylindrical shape. Moreover, detecting means for detecting a position of the magnetic fluid is provided to detect acceleration by detecting a position of movement of the magnetic fluid which is moved in accordance with the acceleration.
However, in the first prior art example, problems are encountered in that since it is designed to prevent an erroneous movement of the weight by utilizing the viscosity of air which passes through the gaps between the weight 23 and the cylindrical guide 24, a high accuracy is required in the sphericity of the weight 23, the cylindricity of the cylindrical guide 24, and the size tolerance between the weight 23 and the cylindrical guide 24. As a result, the machining of the parts is difficult. In addition, since gold plating for rust prevention is required to be applied to the weight 23 to avoid unsatisfactory movement of the weight 23 due to rust caused by the air in contact therewith, the working cost of the sensor is increased.
In the second prior art example, since it is arranged that the impulsive force is transmitted to the magnetic fluid through the medium in order to detect acceleration by the movement of the magnetic fluid, the response of the sensor is degraded. As a result, there is an unsolved problem in that the above-mentioned sensor can not be used as an impulse sensor which is required to be highly responsive so as to be instantly operated upon collision of the automobile to actuate an air bag.